1. Field of the Invention
The present invention is related to an anti-noise method for the direct current brushless motor without using sensor device, and more particularly, related to a anti-noise method utilizing a Back Electromotive Force (BEMF) detection circuit to detect a BEMF induced from the coils of the outer motor and a gain control signal feedback from the phase detection circuit and frequency detector to determine rotation speed and phase of the external motor so as to guarantee that BEMF signal from the direct current brushless motor is able to determine rotation speed and the phase of the external motor.
2. Description of the Prior Art
The technique related to the DC brushless motor in prior art discloses a anti-noise method utilizing a startup circuit to output different startup frequencies and output different driving currents from the control circuit to pass through the driving coil of the external motor and feedback the external BEMF to detect by the detective circuit so as to determine the startup and the operation of the motor in order to guarantee the motor is working properly.
As shown in FIG. 1A, it is a block diagram illustrating that a DC brushless motor system without sensor device. As shown in FIG. 1A, the system includes an external motor 11, a control circuit 12, a output circuit 13, a detective circuit 14, a startup circuit 15, and a switching circuit 17. The startup circuit 15 outputs different driving frequencies square waves to the output circuit 13 and the corresponding output current is outputted to the driving coil of the external motor 11. The driving coil of the external motor 11 will generate the Back Electromotive Force (BEMF) to feedback to the detective circuit 14 and the detective circuit 14 will determine the rotation speed and the phase of the external motor 11 in accordance with the BEMF so as to control the startup and the rotation speed of the motor. When the startup circuit 15 is activated to output the activated frequency signal to the control circuit 12, and the signal is transformed to be a six steps driving control signal shown in FIG. 1B, into the output circuit 13. The current of the driving coil of the external motor 11 is accordance with the phase difference of the six steps driving control signal, and the rotation speed and the phase of the external motor 11 is determined by the current of the driving coil.
FIG. 2 is a view illustrating a conventional BEMF detector without sensor device. The detector includes a three-phase induction motor 11, a BEMF sampler 26, a normal phase comparator 202, a reverse phase comparator 204, a voltage reference (Vth), a normal phase switch 210, a reverse phase switch 212 and a output switch 214. When the motor system is activated, the six steps driving control signal is inputted into the output circuit 13 and three-phase current is outputted from the output circuit 13 to the three-phase motor coil 11 so as to generate the BEMF by the current of the three-phase motor coil 11.
The BEMF is inputted in the positive end of the normal phase comparator 202 and the negative end of the reverse phase comparator 204. The voltage reference (Vth) 206 is connected to the negative end of the normal phase comparator 202 and the positive end of the reverse phase comparator 202. When the phase of the BEMF outputted from the BEMF sampler is between 0˜180 degree, the phase value is more than the positive voltage reference, the voltage outputted from the normal phase comparator 202 is high voltage reference. At final, the output of the normal phase comparator 202 and the output of the reverse phase comparator 204 will pass through the normal phase switch 210 and the reverse phase switch 212, and then merge together to be a square wave signal in the output switch 214. Therefore, the square wave outputted from the output switch 214 will inputs to the detective circuit 14 so as to detect the rotation speed and the phase of the external motor 11.
According the description above, The BEMF generated at the three-phase motor coil is detected by the BEMF sampler 26. The high voltage reference is sampled by the normal phase comparator 202 and the reverse phase comparator 204 and inputted into the detective circuit 14 for determining the rotation speed and the phase of the external motor 11. However, the motor driving circuit system in initial rotation and high speed rotation will generate different noise. If the sample voltage of the BEMF is sampled in accordance with the same voltage in different rotation, the system would make a mistake to make a wrong decision so as to cause the malfunction of the system.
According to the motor driving circuit system described above, a different motor driving circuit is provided in the present invention for utilizing at different rotation mode to provide different sample voltage of the BEMF. Therefore, the noise ratio of the BEMF is better and the rotation speed and the phase of the external motor are detected properly so as to achieve a better stability of the system.